1. Technical Field of the Invention
This invention relates to a process for removing, or at least substantially reducing, metal carbonyls in a gaseous stream, such as a synthesis gas stream, by making use of a lead oxide, PbO, on a gamma alumina sorbent.
2. Background of the Invention and Prior Art
Metal containing catalysts are used to catalyze many important industrial processes, such as ammonia synthesis, methanol synthesis, and Fischer-Tropsch synthesis of hydrocarbons and oxygenated hydrocarbons. The metal catalysts are susceptible to catalyst poisoning by even minute amounts of metal carbonyls, such as iron carbonyl, cobalt carbonyl and nickel carbonyl, when found in the gas streams used in these processes. When carbon monoxide is present in any appreciable amount in the gas stream under conditions of relatively low temperatures (about 25.degree. to 100.degree. C.) and particularly at elevated pressure, contact with iron, nickel or cobalt metals will cause the formation of metal carbonyls. Iron carbonyl is often formed by the reaction of carbon monoxide with steel materials in the process equipment. Also, metal carbonyls can form when the gases are transported in steel containers.
During synthesis processes, the process catalysts themselves are not usually vulnerable because the processes are run under conditions which are not conducive to the formation of metal carbonyls. Problems arise, however, in the stages of the process preceding the reaction zones. At this stage, conditions often permit the formation of metal carbonyls upstream of the reaction steps. When these carbonyls reach the reaction zone, they poison the process catalysts, notwithstanding the proper control of the conditions in the reaction zones.
The synthesis of ammonia involves the use of a synthesis gas from which the carbon monoxide has been removed and which has been adjusted to a ratio of 3 parts of hydrogen and 1 part of nitrogen. Processing with a catalyst at high temperatures and pressures yields ammonia.
Methanol is a major petrochemical product having a great variety of uses and many believe that it will become an important energy carrier in the future. The most important source for methanol production nowadays is natural gas. When natural gas is used as a feed in methanol production, the natural gas feed is freed from any sulphur impurities by catalytic or adsorption processes before entering one or several reformer stages in which the natural gas is transformed into a synthesis gas of suitable composition for methanol production. Methanol is produced at pressures of about 60 to 100 bar and temperatures of about 200.degree. to 300.degree. C. in the presence of a catalyst. Even relatively small amounts of iron carbonyl, Fe(CO).sub.5, of about 5 ppm in the feed gas have been found to poison the catalysts used in the methanol synthesis.
The third process of particular interest is the Fischer-Tropsch synthesis of hydrocarbons and oxygenated hydrocarbons. The hydrocarbon product from this process is transformed into hydrocarbon fuel, in particular high quality diesel fuels and blending components for other fuels. This process has therefore been in the focus of interest for quite a number of years, but due to technical and economical reasons has not until now been deemed economically viable, except under particular economic and political conditions. Due to technical developments in recent years, the Fischer-Tropsch synthesis is now on the brink of being declared economically feasible in situations where, for instance, large supplies of natural gas are available at low costs in remote areas and an effective method is needed for converting the natural gas into a synthetic crude capable of being transported to facilities located near larger fuel markets and refineries.
As for the production of methanol, natural gas for the production of Fischer-Tropsch products is first converted into a synthesis gas containing hydrogen and carbon monoxide in a suitable ratio. The synthesis gas is produced by one of several processes, some depending on the source of the hydrocarbon. Both natural gas and clean coal are now sources for synthesis gas.
The processes used to synthesize syngas from natural gas are (1) steam-methane reforming, (2) partial oxidation using oxygen and methane feed, and (3) autothermal reforming of methane with oxygen or air or combinations thereof. The coal-source synthesis gas is produced from partial oxidation of coal using oxygen.
All of these processes have the potential for producing syngas which can be contaminated with iron carbonyl in downstream pipe, vessels, and other process equipment. The Fischer-Tropsch synthesis is carried out in a subsequent step in which hydrogen and carbon monoxide react at pressures of about 10 to 50 bar and temperatures in the range of about 150.degree. to 300.degree. C., in the presence of a suitable catalyst, which normally comprises iron or cobalt dispersed on a suitable support.
In order to eliminate or reduce the risk of catalyst poisoning by metal carbonyls in the above-mentioned processes and similar processes, it would be desirable to effectively remove or substantially reduce the metal carbonyl from the respective feed gas streams to the lowest possible level before converting the feed gas streams as described.
Suggestions have been made in the art to pretreat synthesis gas to remove metal carbonyls by making use of materials including molecular sieves, alumina, Cu, CuO, and ZnO.
Thus, Dvorak, L. et al. (Chemical Abstracts, Vol. 96 (1982), Abstract No. 164.903e) have attempted to remove residual quantities of sulphur compounds and/or iron pentacarbonyl, Fe(CO).sub.5, from gas mixtures by contacting the gases with spent catalysts, the main components of which were Cu and/or CuO and ZnO. Only small amounts of iron carbonyl were sorbed from the gases.
The problems with these suggestions are two fold. First, the adsorbents, such as, molecular sieves and alumina have a low adsorption capacity, and second, the Cu and CuO sorbents have the draw back of being hydrogenation catalysts. When used, they can convert some of the syngas to methane and alcohols. This of course is undesirable.
It has also been proposed to use copper coated tubes in the process plant to avoid reaction of the synthesis gas with iron and nickel with formation of iron and nickel carbonyls. However, such a solution to the problem is unpractical and expensive. The normal procedure to reduce the potential formation of Fe(CO).sub.5 in syngas streams is to use austenitic (18/8) stainless steel pipes and vessels. But this option may not be practical.
U.S. Pat. No. 3,782,076 to Norman L. Carr et al., which issued on Jan. 1, 1974, teaches the use of an oxide of lead dispersed on an alumina support in a process for reducing the arsenic content of a gaseous hydrocarbon stream. While the feed gas described in the Carr et al. patent might typically contain relatively small quantities of carbon monoxide (0.2 to 3.4 Vol. %), there is no teaching that metal carbonyls would be formed. Further, it is believed that detectable levels of metal carbonyls would not be formed with the low levels of carbon monoxide described in the Carr et al. patent. The levels of carbon monoxide are greater in the feed streams used in Fischer-Tropsch, methanol and ammonia syntheses than those described in the Carr et al. patent. Thus, the formation of metal carbonyls, while not a problem in the method described in the Cart et al. patent, is a serious problem in other processes.